Electrodeless fluorescent lamps utilizing an inductively coupled plasma were found to have a high efficacy and lives which are longer than conventional fluorescent lamps that employ hot cathodes. The plasma that generates visible and UV light is induced in a glass (or quartz) envelope filled with inert gas such as argon, krypton at pressure of 0.1-2 torr and mercury vapor. To generate such a plasma at a frequency of 13.56 MHz, electrodeless lamps employ an induction coil positioned near the lamp envelope. The prior art teaches three basic approaches of coupling the induction coil and the lamp plasma at a frequency of 13.56 MHz.
The most simple coupling method is wrapping the induction coil around the envelope which was disclosed in U.S. Pat. No. 5,013,975 by Ukegawa et al. This approach provides good coupling between the coil and plasma, but has at least three disadvantages. The coil is exposed and radiates electromagnetic waves and therefore the lamp needs screening (mesh, special screening wire around the lamp, etc.). The lamp operation is very sensitive to the fixture's shape and size due to good capacitive coupling between the outside coil and the fixture. The lamp with a coil outside is not aesthetically attractive.
Another approach used for electrodeless lamps operated at a frequency of 13.56 MHz was suggested in U.S. Pat. No. 4,010,400 by Hollister, and U.S. Pat. No. 5,621,266 by Popov et al. The induction coil was inserted in a reentrant cavity located along the envelope axis. Such an arrangement provides a good coupling between the coil and the toroidal or cylindrical-shaped plasma. The coil is screened by the plasma so the introduction of the fixture does not affect the lamp performance. This approach has two disadvantages. The introduction of the reentrant cavity reduces the volume of the envelope filled with the plasma which results in a decrease of the lamp efficacy. Heating of the cavity walls and the adjacent coil by the plasma radiation requires special means for cooling the coil and walls. A slotted aluminum cylinder inserted in the reentrant cavity and welded to the lamp base is disclosed as a cooling means in U.S. Pat. No. 5,621,266 by Popov et al., and U.S. Pat. No. 5,698,951 by Maya et al. The same cylinder works also as a Faraday shield between the coil and the plasma, thereby reducing the energy of ions bombarding the cavity walls and, hence, improving the lamp maintenance.
The third approach that is suitable for the operation at 13.56 MHz is based on the utilization of a spiral coil attached to the bottom of the envelope as it is disclosed in U.S. Pat. No. 5,349,271 by Ron van Os et al., and U.S. Pat. No. 5,500,574 by Popov et al. This lamp provides good coupling between the coil and the plasma and does not cause the overheating of the coil and envelope walls adjacent to the coil. To increase the lamp light output, U.S. Pat. No. 5,500,574 teaches coating the bottom of the envelope with a reflective material. However, such approach also has a drawback in that it is difficult to manufacture lamps with a large bottom diameter, which restricts the lamp size.
The decrease of the driving frequency, f, from 13.56 MHz to 2.65 MHz, requires the increase of the magnetic field in the plasma, B.sub.pl, that generates inductively coupled electric field in the plasma, E.sub.pl. The increase of B.sub.pl could be achieved by the increase of the coil current, I.sub.coil, or by the increase of the medium magnetic permeability, .mu..sub.eff. The increase of I.sub.coil leads to the increase of coil power losses, P.sub.loss, EQU P.sub.loss =(I.sub.coil).sup.2 R.sub.coil,
where R.sub.coil is the resistance of the coil.
The increase of coil power losses reduces lamp power efficiency and, hence, lamp efficacy. Therefore, to keep the lamp efficacy high it is necessary to increase B.sub.pl by the increase of the medium permeability, .mu..sub.eff, by the introduction of a ferrite core.
In electrodeless lamps disclosed in U.S. Pat. No. 4,568,859 by Houkes et al., and U.S. Pat. No. 5,343,126 by Farrall et al., and operated at a frequency of 2.65 MHz, the ferrite core was introduced in the reentrant cavity along the envelope axis. The solenoidal induction coil was wrapped around the ferrite core, thereby substantially increasing the magnetic field in the plasma without sacrifice in the lamp efficiency. However, the ferrite core located in the reentry cavity needs to be cooled and maintained at a temperature below Curie point, T&lt;300.degree. C., which requires a cooling means. Moreover, the increase of lamp power to 100-200 W or higher requires the increase of the envelope diameter and the cavity length that makes cooling of the ferrite core and the coil very difficult.
The alternative approach that does not require coil and ferrite cooling was suggested by Anderson in U.S. Pat. No. 3,500,118, and developed by Godyak et al. in U.S. Pat. No. 5,834,905. The electrodeless fluorescent lamp comprises a closed-loop, tubular lamp ("Tokamak" shape), with one or several toroidal transformer cores being disposed around the lamp and an induction coil of several turns wound on the core. The induction coil is the prime winding and the plasma generated in the closed-loop tube is the second winding.
U.S. Pat. No. 5,834,905 teaches that the lamp tube diameter and the lamp discharge current should be high enough to provide low plasma electric field, E.sub.pl &lt;0.5.sup.V /.sub.cm, and, hence, low discharge voltage. The lower the discharge voltage, the lower the magnetic field needed to maintain an inductively coupled discharge, and, hence, the lower the power losses in the ferrite core. By employing a ferrite core with low power loss (P.sub.f /V.sub.f &lt;0.1 W/cm.sup.3) Godyak et al. achieved 94% power efficiency in a lamp operated at RF power of 150 W and at a frequency of 200 kHz.
However, the Anderson-Godyak approach with a ferrite core has a few disadvantages. The ferrite core is relatively expensive, and it requires special ferrite preparation of two thoroughly polished cuts and brackets to keep these surfaces firmly together. Also, a special strip (or wire) made from conductive material and electrically connected to the matching network must be disposed on the lamp tube to ignite a lamp.
The closed-loop lamp of small size (but without a ferrite core) was described in U.S. Pat. No. 4,864,194 by Kobayashi et al. Patentees disclosed a lamp envelope of two straight tubes connected with two hollow bridges and a box (or tube) with a partition that divides the box/tube in two parts. The lamp employs only a single turn as the induction coil. The one-turn coil is disposed around the outer periphery of the lamp.
As the material for the induction coil, patentees described a copper wire, a copper strip or copper foil. The use of only one coil turn (even of large length and diameter) restricts a coil inductance to small value of 1 .mu.H and lower, and, hence, restricts the operating frequency range. Also, the power loss in the coil increases as the number of turns decreases. For the case when the coil diameter is much larger than coil height, the power loss in the coil is: EQU P.sub.loss.varies.(E.sub.pl).sup.2 R.sub.coil /k f (N.sub.coil).sup.2
Here k is the coupling coefficient between the coil and the plasma and N.sub.coil is the number of turns. Typically, k&gt;0.6 for plasmas at pressure, p&gt;100 mTorr and RF power, P&gt;10 W.
It is seen from the above equation that P.sub.loss decreases as the number of turns increases, P.sub.loss.about.1/N.sub.coil (R.sub.coil increases with N.sub.coil linearly). The operation with the single turn could be efficient (low ratio P.sub.loss /P.sub.lamp) only at high frequency of f&gt;13.56 MHz. When the lamp is operated at lower frequency of 2-3 MHz, and lower, the coil with one turn consumes a considerable amount of RF power, making the lamp inefficient.
The induction coil in the lamp described in U.S. Pat. No. 4,864,194 is positioned outside of the lamp around the tube periphery. This location of the coil was chosen because the lamp was designed as a light source for a copying machine where the light was emitted through the inner surface of the tubes. For lamps used in indoor and outdoor applications, such as high ceilings, streets, malls, tunnels, etc., it is desirable to have the envelope's outer surface free of the coil so the coil does not interfere with the light radiated from the envelope. It is also important to note that the coil disposed outside the lamp has a capacitive coupling with the fixture that affects the lamp operation.